This invention is related to imaging multiple tracks simultaneously using a beam multiplier to form a set of single-mode beams and a spatial light modulator (SLM) to modulate the single-mode beams to form the tracks, such that diffraction-limited optics can be used to focus the beams.
It is known to use a multichannel modulator to simultaneously form a plurality of beams to form an image on a recording medium. Such multichannel imaging is particularly advantageous in the printing industry for film and plate imaging.
Recently, the introduction of SLMs using Micro Electromechanical Systems (MEMs) has provided for imaging using a relatively large number of individually modulated beams. MEMS are made of arrays or matrixes of reflective elements (mirrors) made of micromachined silicon that can be steered by electrostatic forces.
A relatively large number of modulated beams enables lowering the rotational speed of a polygon in the case of a stationary internal drum imaging system using a polygon, and lowering the rotational speed of a drum in the case of an external drum imaging system.
Multichannel imaging is also useful in other fields of application.
FIG. 1A shows in simplified form one example of a prior art imaging system that uses a line-shaped laser diode source 101 and a lens to illuminate a multichannel spatial light modulator (SLM) 105. The SLM 105 separates the beam illuminating the SLM into a plurality of individually modulated beams 106 that are imaged using an optical system-shown in simple form as lens 107 onto a light sensitive surface on an image plane 109 that, for example, might be the inside surface of an internal drum imager, or the outside surface of an external drum imager. Not shown are the plurality of signals that modulate the plurality of beams, or the mechanism used to provide relative motion between the beams on the imaging plane, and the light sensitive surface on which an image is being formed. The relative motion may be provided by moving the beam, e.g., using a rotating polygon in an internal drum scanner, or by moving the radiation-sensitive surface of the recording medium, e.g., rotating the drum of an external drum scanner.
FIG. 1B shows in simplified form another example of a prior art imaging system, this one using an array of laser diode sources 111 and an array of lenslets 113 to illuminate an SLM 115, replacing the single line-shaped source 101 and lens of FIG. 1A. The remainder of the system is similar to that of FIG. 1A. That is, SLM 115 separates the beams illuminating the SLM into a plurality of individually modulated beams 116 that are imaged using an optical system 117 onto a light sensitive surface on an image plane 119. FIG. 1B also does not show the mechanism for providing relative motion between the modulated beams hitting the light sensitive material and the light sensitive material itself.
Prior art systems also are known that use a large area laser beam source rather than a line-shaped source. In such a case, the SLM is a two-dimensional array of light modulators.
One of the drawbacks of the prior art using either an array of laser diodes or a large area or line-shaped laser diode source is that the beam is of relatively poor quality. For example, for line-shaped and large-area sources, also known as multimode sources, there is non-diffraction-limited divergence in the direction of the elongated axis. It is known that the design of optical delivery systems for laser systems is highly dependent on the laser's beam quality. The beam quality, given by the parameter M2, ranges from 1 for a diffraction-limited TEM00 Gaussian laser beam, to several hundred for a distorted, poor quality beam. As a result, the optics of a prior art imaging system needs to be carefully designed with tight tolerances and be of high quality. This increasing the system costs and decreasing the reliability. Often, for example, an autofocus system is used to overcome the shallow depth of focus of the optical system, e.g., lens 107 or 117, used in prior art systems.
It is desirable to use SLMs to provide for multichannel imaging, yet it is also desirable to maintain diffraction-limited beam properties. This leads to increased depth of focus and enhanced imaging quality in comparison to conventional imaging optics involving SLMs.